Effect of pore-forming/hydrophilic additive anchorage on the mesoporous structure and sieving performance of a blended ultrafiltration (UF) membrane

Abstract

Pore forming agent or hydrophilic additives are necessary for the optimization of membrane pore structure and surface physicochemical properties. Although it has been demonstrated that the blending additives tends to locate around membrane pore walls due to segregation and this would further leach out in practical operations, the impact of additives fate on membrane pore structure as well as sieving performance is almost overlooked in literatures. In this study, five kinds of Pluronic copolymers with different molecular architectures were employed for casting PSF UF membranes, and the fate of these copolymers in membrane matrix was explored by combing FTIR and XPS spectra with TG analysis. More importantly, the evolutions of membrane mean pore radius (r*) and radius distribution (σ) after additive leaching were quantitatively investigated based on the modified Steric Pore Model (SPM). In passive solute sieving experiments, the increment of r* and σ possessed positive correlation with the degree of polymerization (DP) of EO segments within the corresponding additive. The results of thermoporosimetry and saline filtration experiments further evidenced that such a correlation should be attributed not only to the steric hindrance of EO segment, but also to its hydration ability. Moreover, the intrinsic polymer geometry (rST) was also a crucial factor for understanding the evolutions of membrane pore structure during additive leaching. Especially for membranes having small rST, both the isopropanol (IPA) treatment and poly-dopamine (PDA) decoration experiments suggested that pore enlargement and the generation of new small cavities would occur simultaneously during additive leaching, making the prediction complicated. Finally, we proposed that the relative synthetic aperture (RSA) could serve as a complementary indicator for understanding and describing the pore enlargement behavior of a blended UF membrane. In an industrial point of view, it also offers a new way to evaluate and predict the long-term performance and stability of a blending modified porous membrane.